The invention relates to a power supply for brightness control of a gas-discharge tube, and particularly to a power supply that provides a combination of frequency transitioning and pulse-group modulation for controlling the brightness of a gas-discharge tube.
It is desirable to control the brightness of a neon sign or other gas-discharge tube application. Controlling the brightness of a gas-discharge tube requires some sort of variable power source to drive the tube. The power source is typically one of two types: a xe2x80x9ctransformerxe2x80x9d type power source, or a xe2x80x9cpower supplyxe2x80x9d type power source. The xe2x80x9ctransformerxe2x80x9d type power source steps up the utility voltage, and drives the gas-discharge tube at utility frequency (e.g., 50 or 60 Hz). The xe2x80x9cpower supplyxe2x80x9d type power source rectifies the line voltage to form DC rail voltages, inverts the rail voltages at relatively high frequencies (typically 20-100 kHz), and drives a small step-up transformer that drives the tube. The present invention deals with a gas-discharge tube having a xe2x80x9cpower supplyxe2x80x9d type power source.
Numerous methods have been used in an attempt to dim a gas-discharge tube powered from a power supply. Some methods attempt to reduce the energy delivered to the tube on a continuous basis by reducing the DC rail voltages applied to the inverter. This and similar methods suffer from a common disadvantage; when dimmed, the center of large signs including the gas-discharge tube becomes dimmer than the sections electrically close to the incoming power.
One dimming method that gives the greatest range of dimming, with no significant difference in intensity along the length of the tube, is pulse group modulation (xe2x80x9cPGMxe2x80x9d). For PGM and as shown in FIG. 1, the inverter is operated at full input voltage and optimum frequency (e.g., 20 kHz) for a first interval 15 of a time period 5 (i.e., a first group of pulses 10 is generated for a first interval 15). The inverter is then xe2x80x9cOFFxe2x80x9d for a second interval 25 of the time period 5 (i.e., no group of pulses 20 is generated in the second interval 25). The result is groups of drive pulses being delivered to the transformer and to the tube load. The ON and OFF pulsing is continuously performed at a sufficiently high repetition rate to prevent the perception of flickering (about 100-200 Hz). The overall repetition rate is kept constant, while the lengths of the first and second intervals 15 and 25 are varied to implement dimming. The tube is at full intensity when the ON interval 15 occupies the entire time period 5, and the tube is off when the OFF interval 25 occupies the entire time period 5. In between lies a smooth range of dimming from off to fully bright.
Pulse group modulation suffers from one major drawback. The step-up transformer oscillates at the pulse group repetition rate, producing a loud buzz. A subtler drawback of PGM dimming is that, at lower brightness levels, the tube may extinguish and re-ignite with each pulse group 15. This continuous re-ionization generates radiation electromagnetic interference (EMI).
One prior art method used to combat the above problems is frequency-shift-key (FSK) dimming (see FIG. 2). FSK dimming entails producing a first group of pulses 35 for a first interval 40 of a time period 45 (referred to as the xe2x80x9cONxe2x80x9d portion or mode), ramping to a higher pulse frequency during a second interval 55, producing a second group of pulses 60 for a third interval 65 (referred to as the xe2x80x9cOFFxe2x80x9d portion or mode), and ramping down to the frequency of the first group of pulses 35 in a fourth interval 75. The transformer and tube are continuously driven, but with a much lower energy transfer efficiency during the xe2x80x9cOFFxe2x80x9d portion 60. By varying the amount of time spent in the normal high efficiency xe2x80x9cONxe2x80x9d mode 45 and the low efficiency xe2x80x9cOFFxe2x80x9d mode 55, the sign can be progressively dimmed. Also, since the transformer is continuously driven, the audible noise generated by the pulse group repetition is dramatically reduced.
FSK dimming suffers from one major drawback. The continuously changing drive frequencies generate a wide spectrum of noise, making EMI filtering difficult. However, since FSK dimming continuously drives the tube, it is always ignited, and reignition EMI is not a concern.
The invention provides a power supply connectable to a power source and to a gas-discharge tube. The supply includes a transformer having a primary winding and a secondary winding. The tube is connectable across the secondary winding. The power supply further includes first and second switches that receive first and second drive signals, respectively, and switch power to the primary winding. The power supply further includes a resonant circuit interconnected to the first and second switches, and a controller interconnected to the first and second power switches. The controller is operable to generate the first and second drive signals for a time period. For a first time interval of the time period, the controller generates the first and second drive signals at a first frequency, and transitions the first and second drive signals from the first frequency to a second frequency. For a second time interval of the time period, the controller generates the first and second drive signals at the second frequency. For a third interval of the time period, the controller ceases generation of the first and second drive signals.
The invention also provides a method of controlling the brightness of a gas-discharge tube. The method includes providing a power supply having a resonant circuit, establishing a time period, and generating a varying signal having a varying frequency. The generating of a varying signal includes: for a first time interval of the time period, transitioning the varying frequency from a first frequency to a second frequency; for a second interval of the time period after the first interval, generating the varying signal at the second frequency; and, for a third interval of the time period, ceasing generation of the varying signal. The method further includes providing the varying signal to the resonant circuit.
As was stated earlier, for PGM, the gas-discharge tube may de-ionize between ON intervals when the power source ceases generation of drive signals for extended OFF intervals. De-ionizing the tube results in the tube re-ionizing at the beginning of each pulse group. Since the strike voltage of the tube is typically much higher than the run voltage of the tube, continuous tube re-ionization produces a steady stream of high voltage re-strikes in the tube. High voltage breakdown is more likely for the tube because of the frequent presence of high voltage on the output of the power supply. Consequently, greater high voltage insulation is required, which increases the cost of the power source.
The present invention attempts to overcome the high voltage restrike. The resonant circuit of the power supply operates best at approximately a resonant frequency (e.g., 20 kHz). When a varying signal greater than the resonant frequency is provided to the resonant circuit, the ability of the resonant circuit to transfer energy is compromised. As a result, the voltage at the secondary winding of the power supply""s transformer is limited. In one embodiment of the invention, the power supply of the invention starts each pulse group at a higher than normal drive frequency, and slowly transitions the frequency down toward the normal operating frequency. As the frequency drops, the voltage at the secondary of the transformer increases. When a sufficient voltage is reached, the gas ionizes in the tube. Since the tube voltage is increased slowly, the tube is coaxed into conduction at the lowest possible voltage, minimizing the voltage present at the output leads. The voltage applied to the load is limited at the beginning of each pulse group and, thus, is limited at each re-ionization of the tube. This reduces or eliminates the need to change the physical construction of the transformer to accommodate higher voltages. Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.